1. Field of the Invention
This invention relates to transfer switch systems for switching between alternate power sources for loads connected to a load bus and in particular to such a system including a communications network for coordinating transfers between sources with operation of the loads.
2. Background Information
Transfer systems for supplying alternate electric power sources to specified loads are well known. Often such systems provide power from a commercial power source and an auxiliary power source such as a local, engine driven generator. Such transfer systems are widely used in applications such as, for example, hospitals, computer installations and industrial processes which require continuous power. Many of these transfer switches effect an open transition between the two power sources. That is, the load bus is disconnected from the one source before it is connected to the other source in order to avoid the problems associated with interconnection of unsynchronized sources. This results in interruption of power to the loads. Often the auxiliary power source is not capable of supplying the large in-rush currents associated with simultaneous reenergization of a number of loads such as motors. Typically in such a situation, the loads are reenergized sequentially. The transfer switch includes a separate timer connected by separate wiring to each load. There is no indication to the transfer switch that the loads have responded to the restart signal.
Switching between power sources with an open transition creates special problems when the loads include elevators. Typically, codes require that the elevators be parked at a floor with the doors open before a discretionary transfer between power sources is made. The usual solution is to send a signal to each elevator over separate dedicated leads commanding the elevator to park with its doors open. The transfer switch delays the transfer for a period of time presumed to be long enough to permit the elevators to comply. There is no indication that the elevators have responded.
Many utilities today offer preferential rates to customers who agree to terminate or reduce power usage within a specified time of receipt of notice of the need for a power reduction. Typically, this notification is processed manually to implement the power reduction.
There is a need for an improved transfer switch system with improved coordination between the transfer switch and the loads.
There is also a need for simplifying and control of the loads by the transfer switch and making it easier to accommodate different load configurations.
There is also a need for providing feedback to the transfer switch of the response of the load to commands from the transfer switch especially where the loads are elevators.
There is also a need for an arrangement for automatically responding to notice from a supplier of commercial power to reduce power usage in order to take advantage of preferential rates.